This invention pertains to a system for automatically adjusting the concentration of any material carried in a body of fluid and is particularly useful for maintaining a predetermined concentration of toner material suspended in a liquid carrier for use in a printing system of a type wherein toner particles become adhered to paper or other receiving material so that during printing the concentration of toner particles in the liquid body diminishes simply by consumption of very small amounts of the toner from the liquid body with each printing.
Heretofore, the problem of maintaining a proper level of darkness in a printing system has been typically solved simply by inspection of the quality of the print which is being produced at a given time. If the print is too light then additional toner concentrate is added by the operator of the machine until the darkness is appropriately established.
The foregoing manner of adjusting the darkness level of print in a toner printing system suffers from a number of drawbacks and accordingly there has been a need for a system in which the toner remains automatically properly adjusted to an appropriate level of darkness and in which the darkness level can be increased or decreased as desired without upsetting the manner of operating the system.